It is important to have accurate and useful biomechanical models of human movement in order to develop rational surgical and rehabilitative strategies. This proposal is designed to provide novel, primary data that will enable rigorous static and dynamic biomechanical models of human movement to be developed. We opine that current efforts aimed at developing human models are hampered because (1) a solid primary database of human muscle architecture in the lower extremity are lacking, (2) the operating range of almost all human lower extremity muscles has never been measured, (3) the scaling relationships between cadaveric and human muscles are not known and (4) primary human muscle literature data are scattered as mean values among published studies. In light of these severe limitations, it is not surprising that efforts to model human movement and to evaluate and plan surgical interventions have been hampered. We propose experiments that will provide a wealth of primary data to the scientific community and thus stimulate the development of realistic human musculoskeletal models using the following specific aims: (1) Comparing human skeletal muscle physiological properties that are measured directly in the operating room with muscle function predicted from those same muscles obtained after fixation; (2) Using intraoperative laser diffraction to measure the sarcomere length-joint angle relationship of living human muscles (and thus their functional operating ranges), for those muscles most commonly modeled in the lower extremity, (3) Implementing noninvasive magnetic resonance imaging (MRI) to create accurate, subject-specific representations of muscle function., and (4) Establishing an internationally-accessible, definitive database of primary data of human lower extremity muscle architectural properties and their associated tendons. This data, once available to the biomechanical community will propel this field to the next level and enable development of meaningful new surgical procedures and objective methods of evaluating human musculoskeletal function.